The present invention relates to the field of solid state electronics and, more particularly, to a method of electric discharge processing of thin films.
In the semiconductor field, it is often desirable to selectively remove or otherwise process thin films in well defined patterns. Selective removal of films in the manufacture of thin film monolithic solar cells is described in: Hanak "Monolithic Solar Cell Panel of Amorphous Silicon", Solar Energy 23:145-147 (1979); Borden U.S. Pat. No. 4,278,473; Tyan et al. U.S. Pat. No. 4,315,096; and Morel et al. U.S. patent application Ser. No. 495,198, filed May 16, 1983. These works relate to specific monolithic constructions wherein thin film layers are patterned to produce a number of individual cells connected electrically together. The films making up the cells are patterned by a variety of techniques, including mechanical scribing, laser scribing and wet chemistry (photoresist) techniques.
None of the prior patterning techniques has proven both reliable and cost effective in production. Mechanical scribing is a relatively crude process in which a hard scribing tool contacts a film surface. The tool removes a wide path of material and tends to wear rapidly when relatively hard films, such as tin oxide (TO), are scribed. Laser scribing produces a thin, well-defined line in two dimensions, but is difficult to control depthwise. Thus, it is difficult to selectively remove one layer of a thin film structure without penetrating the other layers. Laser scribing also requires a large capital investment. Such scribers typically have a single scribing beam and are not well suited to high volume production. The wet chemical approach to patterning thin films is probably the least desirable method in solar cell manufacture. It is costly and time-consuming to perform, due to the number of steps required, and can contaminate a cell structure.
Other methods have been proposed to interconnect active portions of monolithic solar cells. According to one such method, a number of relatively tall contact grid lines are positioned beneath a photoactive layer to electrically connect the front of one cell to the back of an adjacent cell. Another method involves the application of heat with a laser to crystallize or dope a well defined path through a photoactive layer. Both methods are described in Morel et al. U.S. patent application Ser. No. 495,198, filed May 16, 1983. However, the former method tends to reduce the active area of the module, and the laser method suffers from the limitations discussed above. The laser method is also cumbersome because the module must be moved relative to the laser.
Therefore, it is desirable in many applications to provide a method for selectively processing thin films in well-defined patterns without the use of wet chemistry or a laser source.